Move _StreamSinkImpl from dart:io to dart:async as StreamSinkAdapter.

sdk/lib/io/websocket_impl.dart

Index: sdk/lib/io/websocket_impl.dart
diff --git a/sdk/lib/io/websocket_impl.dart b/sdk/lib/io/websocket_impl.dart
index ee56378d788a279caf39372d7084f05931c712b9..5ccb4bd2e5dad7d9026728671789cc695793b9a9 100644
--- a/sdk/lib/io/websocket_impl.dart
+++ b/sdk/lib/io/websocket_impl.dart
@@ -864,7 +864,7 @@ class _WebSocketImpl extends Stream implements WebSocket {
   _WebSocketImpl._fromSocket(this._socket, this.protocol,
                              [this._serverSide = false]) {
     _consumer = new _WebSocketConsumer(this, _socket);
-    _sink = new _StreamSinkImpl(_consumer);
+    _sink = new StreamSinkAdapter(_consumer);
     _readyState = WebSocket.OPEN;
 
     var transformer = new _WebSocketProtocolTransformer(_serverSide);